The goals of this project are to define the stimuli that induce the development of memory T cells, and to identify conditions that promote the expansion and maintenance of memory cells. The principal experimental systems to be used are T cell receptor (TCR) transgenic mice in which the majority of T cells are CD4+ cells specific for defined peptides, and non- transgenic mice immunized with model protein antigens. The results obtained from this project will be used to optimize protective immunity induced by DNA vaccines (project 1) and viruses (project 2). The specific aims of this project are the following. 1. Development, in vivo survival, and stability of effector and memory T cells: T cells expressing a transgenic TCR will be activated with antigen in different ways, including protocols designed to generate T/H1 and T/H2 populations. The survival and functional stability of these activated cells will be studied by adoptive transfer into syngeneic normal or T- deficient recipients. In particular, we will compare the in vivo behavior of adoptively transferred naive and activated cells, and correlate functional profiles of recovered cells with their surface phenotypes. These experiments will establish the requirements for inducing memory cells, and the basis characteristics of these cells in vivo. 2. Role of antigen in maintenance and functions of memory T cells: The influence of different forms of antigenic stimulation on the relative induction of memory and effector cells will be examined. The role of antigen in maintaining memory populations in vivo will be studied, and the responsiveness of naive, recently activated and memory cells to antigen will be compared. 3. Roles of cytokines and costimulators in maintenance and functions of memory T cells: The requirement for specific cytokines and costimulators in the maintenance and stability of memory cells in vivo will be examined using specific antagonists and knockout mice. The ability of these agents to influence T cell memory will be studied with recombinant cytokines and double transgenic mice that co-express TCRs with cytokines or costimulators. Thus, this project will provide fundamental information about the conditions that induce and maintain memory T cells specific for defined antigens, and will serve as the basis for optimizing protocols for inducing long-lived protective immunity.